kbuild: fix comment about dst of headers_{install, check}_all

Commit 61562f98 ("uapi: export all arch specifics directories")
changed the dst from asm-<arch> to arch-<arch> for headers_install_all
or headers_check_all.  Update the comment.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>